1. Field of the Invention
The present invention relates to a semiconductor device including an oxide semiconductor and a manufacturing method thereof.
In this specification, a semiconductor device generally means a device which can function by utilizing semiconductor characteristics, and an electrooptic device, a semiconductor circuit, and an electronic device are all semiconductor devices.
2. Description of the Related Art
A technique for manufacturing a transistor or the like by using an oxide semiconductor film for a channel formation region and applying it to a display device has been attracting attention. Examples of such a transistor include a transistor in which zinc oxide (ZnO) is used as an oxide semiconductor film and a transistor in which InGaO3(ZnO)m is used as an oxide semiconductor film. A technique for manufacturing such a transistor including an oxide semiconductor film over a light-transmitting substrate and applying it to a switching element or the like of an image display device is disclosed in Patent Documents 1 and 2.
Patent document 3 discloses a structure of a transistor including a source electrode and a drain electrode in contact with an oxide semiconductor layer, a gate electrode overlapping with the oxide semiconductor layer, and a gate insulating film provided between the oxide semiconductor layer and the gate electrode, in which the source electrode and the drain electrode each include a first conductive layer, and a second conductive layer having a region which extends in a channel length direction from an end portion of the first conductive layer.
In Patent Document 4, a structure in which gate electrodes are formed below and over an oxide semiconductor layer is disclosed.